Abstract

Populus tremuloides (trembling aspen) recently experienced extensive crown thinning, branch dieback, and mortality across North America. To investigate the role of climate, we developed a range-wide bioclimate model that characterizes climatic factors controlling distribution of aspen. We also examined indices of moisture stress, insect defoliation and other factors as potential causes of the decline. Historic climate records show that most decline regions experienced exceptionally severe drought preceding the recent episodes. The bioclimate model, driven primarily by maximum summer temperatures and April–September precipitation, shows that decline tended to occur in marginally suitable habitat, and that climatic suitability decreased markedly in the period leading up to decline in almost all decline regions. Other factors, notably multi-year defoliation by tent caterpillars (Malacosoma spp.) and stem damage by fungi and insects, also play a substantial role in decline episodes, and may amplify or prolong the impacts of moisture stress on aspen over large areas. Many severely affected stands have poor regeneration potential, raising concerns that increasing aridity could ultimately lead to widespread loss of aspen forest cover. The analysis indicates that exceptional droughts were a major cause of the decline episodes, especially in the drier regions, and that aspen is sensitive to drought in much of its range. Coupling the bioclimate model with climate projections suggests that we should expect substantial loss of suitable habitat within the current distribution, especially in the USA and Mexico.

Plain Language Summary

Trembling aspen is the most wide-ranging tree in North America and plays an important role in the forest ecosystems of many areas where it is found. But in the first decade of the 21st century, aspen trees lost branches and died at high rates in some of these areas (called “decline”). This is different from normal losses of these trees due to natural processes when trees mature or when forest species change. This study looked at this troubling pattern across aspen’s entire North American range, in the context of the climate “niche” in which the trembling aspen fares well and the history of climatic changes. It found that almost all areas where the trees declined had suffered extreme drought in the years leading up to the decline. In some areas, leaf damage by tent caterpillars and stem damage by fungi and insects played a role, probably worsening the effects of drought. Areas where the conditions for aspen were marginal were particularly susceptible to decline. The study also projected the future of aspen forests based on these findings and on climate projections. It showed that we can expect future loss of habitat for aspen in many areas of its range, especially in the United States and Mexico.